Inorg. Chem. 1992. 31. 2654-2655
2654 Scheme 11
calculated stabilities for all the steps exclude steric and strain instabilities expected on the actual surface. In place of Scheme I we propose Scheme 11. Here CO binds to the acylated Rh fragment on the right to form 111. However, the Rh-Rh bond in 111 is broken so it rearranges to IV with CO
nearly colinear with the Rh-Rh axis. Species IV rearranges to V which, like species I1 that was proposed in ref 2 for Scheme I, has two CO bound to the right-hand Rh, but one of them binds weakly through 0 to the left-hand Rh. The Rh-Rh distance in I1 is large, which is consistent with the EXAFS result, and the C O IR spectrum will show splitting as seen in ref 3. The activation energy for the C O insertion reaction, going from V to IV,and its reverse, has not been calculated but, based on a theoretical study involving another low-coordinate transition metal cation,' I it should be small. Acknow-t. We thank the B. F. Goodrich Co. for support of this work through a graduate fellowship. (11) Anderson, A. B.; Yu, J. J . Card 1989, 119, 135.
Notes Contribution from the Gross Chemical Laboratory, Duke University, Durham, North Carolina 27706, and Boron Biologicals, Inc., 533 Pylon Drive, Raleigh, North Carolina 27606
A New and Convenient Synthesis of Sodium Carboxylatotribydroborate, Na2BH3C02,a Boron Analogue of Sodium Acetate
Scheme I Me,NBH2COONa 2
NaAIHZEtZ
*
THF/reflux/l6h
Na2BH3CO0 eo 504b
Scheme I1 0
II
Anup Sood and Bernard F. Spielvogel*
Received May 1 , 1990
b
-
*Towhom correspondence should be addressed at Boron Biologicals, Inc.
NaBHlCH3
+
unidentified species
a
Me3NBH2CH3
a
Acetate ion is a simple yet important species involved in several biological processes,' including the biosynthesis of cholesterol. An isoelectronic and isostructural boron analogue of acetate ion is the carboxylatotrihydrobrate anion, H3BC02- (1). Thus, 1 may have interesting biological properties due to its structural similarities to the acetate ion. 1 is also isoelectronic with the carbonate ion and is commonly called boranocarbonate.2 Malone and Parry,2 while comparing the chemical properties of isoelectronic BH3C0 and COz, synthesized several salts of 1 by reaction of H3BC0 with alcoholic base. They also showed that H3BC0 can be regenerated from salts of 1 by reaction with 85% H3P04. Use of H 3 B C 0 as an acyl ion equivalent has been demonstrated p r e v i o ~ s l y and ,~~~ provides a convenient route for incorporating boron into molecules with easy to acylate functionalities. The resulting species may have potential in boron neutron capture therapy (BNCT). Since salts of 1 are stable for long periods under ambient conditions, these should be convenient solid storage sources for the generation of BH3C0 upon demand. In addition, they may also be of value as selective reducing agents4 (such as found for cyanoborohydride). Despite the potential use of 1, studies of the chemistry and biological activity of this species have been limited due in part to the hazardous nature of its synthesis. The previous synthesis? as mentioned (vide supra), involved the use of H3BC0, which itself is prepared from BzH6and C O under pressure. Not only are all these gases hazardous, but the synthesis also requires use of special equipment. We now wish to report a new and convenient synthesis of sodium boranocarbonate and the results of preliminary studies of its biological activity. Reaction of the sodium saltSof trimethylamine-carboxyborane6 (2) with 2 molar equiv of sodium diethyldihydroaluminate (Aldrich) in refluxing THF yielded 1 in ca. 50% yield' according to Scheme I. The product was easily isolated by filtration under an inert atmosphere. The major byprcduct was the completely
+
BH3CH,
BH3COOX
reduced species NaBH3CH3,which stayed in solution with small amounts of other unknown byproducts. At least 1.5 molar equiv of reagent was necessary to completely consume the starting material under the conditions reported. With lower amounts of reagent (1 -0-1.25 equiv), a small amount of unreacted starting material contaminated the product without increasing the yield (by decreasing the amount of reduction). Free trimethylaminecarboxyborane could also be used as substrate, but it consumed extra valuable reagent by immediate conversion to salt. Attempts to prepare Omethylboranocarbonate, H3BC(0)OMe-, from trimethylamine-carbomethoxyborane,8 Me3NBH2C02Me,by a similar procedure were unsuccessful; instead formation of BH3CH3- was observed. Reaction9 of Me3NBH2COOX (X = H , Na, Me) with several other hydrides (except N a H ) either gave no reaction or formed M+BH3CH3and/or Me3NBH2CH3(Table I). This itself is a new method for the preparation of M+BH3CH3-and has been utilized for the Lehninger, A. L. Biochemistry; Worth Publishers Inc.: New York, 1975. Malone, L. J.; Parry, R. W. Inorg. Chem. 1967, 6, 817-822. Zetlmeist, M. J.; Malone, L.J. Inorg. Chem. 1972, 1 1 , 1245-1247 and references therein. Spielvogel, B. F.; McPhail, A. T.; Knight, J. A.; Moreland, C. G.; Gatchell, C. L.; Morse, K . W. Polyhedron 1983, 2, 1345-1352. 2 was prepared in 97% yield by reaction of Me,NBH2COOH with NaHCO, using a procedure similar to the one reported by Morse et al. (Norwood, V. M., 111; Morse, K. W. Inorg. Chem. 1986, 25, 3690-3693). ' H NMR: 2.60 ppm, S. "B NMR: -8.00 ppm, t, 'JB,H = 94 1 Hz. Spielvogel, B. F.; Wojnowich, L.; Das, M. K.; McPhail, A. T.; Hargrave, K. D. J. Am. Chem. SOC.1976, 98, 5702-5703. Yields up to 66% have been obtained. 'H NMR: 0.73 ppm, q, I J i i g H = 80 1 Hz,and septet, ' J I o , H = 26.7 i 0.21 Hz. "B NMR: -31:l ppm, q, ' J I I=~80 ,~ i 2 Hz. Anal. Calcd for BH3C02Na2: C, 11.57; H, 2.91; B, 10.41. Found: C, 11.26; H, 2.92; B, 9.79. Spielvogel, B. F.; Ahmed, F. V.; McPhail, A. T. Synthesis 1986, 833-835. The reactions were followed by "B NMR spectroscopy.
*
0020-166919211331-2654%03.00/0 0 1992 American Chemical Society
Inorg. Chem. 1992, 31, 2655-2658
2655
Table I. Reaction of Me3NBHzCOOX(X = H, Na, Me) with Various Hydrides
no. of molar equiv
hydride Et.NBH4" NaBH, NaBH4/MeOHI2 K-Selectride (KB[CH(CH3)C2H5]3H)
4 2.5 2
LiAl(O-1Bu)3H NaA1H2Et2 LiAlH,
2 2 1 or 2
conditions results X = Me CH2C12/reflux/64h no reaction' diglyme/reflux/60 h no reactionb diglyme/reflux/ 1 hC no reaction" THF/rt/72 h trace amount of Me3NBH2CH,dremaining unreactedb reflux/ 16 h same results THF/reflux/24 h small amount of Me3NBH2CH3dremaining unreacted Et20/rt/24 h NaBH3CH, Et,O/rt/l h LiBH,CH3 + trace Me3NBH2CH3
NaBH, LiAIH, LiA1(0-'Bu)3H
2 2 1 2
X=H THF/reflux/l.S h Me3NBH,b.e Et20/rt/l h LiBH3CH3,some Me3NBH2CH3 Et20/-78 OC/6 h LiBH,CH,, Me3NBH2COOLi THF/reflux/72 h Me,NBH2COOLi
NaBH, NaH
2 2
Vitride LiAlH,
2 2
2
X = Na THF/reflux/4 h no reactionb decomposition to species withoui B-H bonds, small amount of THF/reflux/48 h Na2BH3CO0and unreacted starting material THF/reflux/l9 h MBH3CH3 THF/rt/24 h MBH3CH3
'Hydride reagent completely decomposed. bSomedecomposition of hydride reagent. 'The reaction mixture was refluxed for 1 h prior to addition of MeOH and then refluxed for another 1 h. dSince Me3NBH2CH3is very volatile, some of it may have escaped during !he reaction. eMe3NBH3 Me3NBH2COONa+ Hz + is probably formed by the displacement reaction of diborane (generated as follows: Me3NBH2COOH+ NaBH, 1/zB2HP)with Me3NBHzCOOH(Na). Since no Na2BH3C00was observed, the displacement product, BH2COOH(Na), must degrade quickly before it can convert to 1.
-
preparation of Me3NBH2CH3and subsequently NH3BH(CH3)CONHEt,l0 the amide of the boron analogue of alanine (the first boron analogue of amino acids with a side chain). With NaH as the hydride source and X = Na, formation of small amounts of 1 was observed along with major decomposition. Formation of 1, MBH3CH3,and in some cases Me3NBH2CH3 depends upon the rate of the following two reactions: (a) displacement of Me3N by H-; (b) reduction of the carbonyl group (Scheme 11). In the case where X = Me (and reduction is observed), the reduction of carbonyl is much faster than displacement of Me3N. Thus, formation of only Me3NBH2CH3(in small amounts) is observed upon reduction with K-Selectride (KB[CH(CH3)C2H513Hor LiAl (O-'Bu),H. With stronger hydride reagents, reaction b is followed by reaction a. The carbonyl group in 2 is quite difficult to reduce, and when NaAlH2Et2is used as the hydride source, both reactions proceed at a similar rate. The carbonyl group of 1, formed by displacement of Me3N from 2, is even harder to reduce not only because of the negative charges on the adjacent atoms but also due to the insolubility of 1 in THF. From the data presented in Table I, it is also clear that the partially reduced species are very susceptible to reduction and are probably reduced as soon as they are formed. In summary, a convenient synthesis of sodium boranocarbonate is described. Not only does it easily make available large amounts of 1 for biological studies and for selective reduction studies, but it also provides an easy source of BH3C0 for incorporation of boron into potential BNCT agents. In pharmacological studies" in murine model screens, compound 1 has shown significant hypochole~terolemic~~ and anti-
inflammatoryis activity. These data will be published separately. Acknowledgment. This research was supported by the Army Research Office. Registry No. Na2(l), 17363-08-5; 2, 103904-11-6; NaAlH2Et2, 17836-88-3; Me3NBH2CH, 52920-77-1; NaBH,CH,, 141344-69-6; LiBH3CH3,52950-75-1; Me,NBH,, 75-22-9; LiBH,CH,, 52950-75-1; Me3NBH2COOLi, 141344-70-9; Me,NBH,COOMe, 9 1993-52-1; Me3NBH2COOH,60788-33-2; Et,NBH,, 17083-85-1; NaBH4, 1694066-2; KB[CH(CH,)C2H5],H,54575-49-4; L~AI(O-'BU)~H, 17476-04-9; LiAIH4, 16853-85-3; vitride, 22722-98-1. Supplementary Material Available: Text giving a detailed synthesis of sodium carboxylatotrihydrobrate (1 page). Ordering information is given on any current masthead page.
Contribution from the Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 5 13, 5600 MB Eindhoven, The Netherlands, and Debye Research Institute, Department of Metal-Mediated Synthesis, University of Utrecht, Padualaan 8, 3584 CH Utrecht. The Netherlands
Oxidation State of Platinum in Oxidative-Addition Reactions and #-Iz Products from Dihalogen Reactions with Organoplatinum(I1) Complexes, As Inferred from Monochromatic X-ray Photoelectron Spectroscopy J. C. Muijsers,+ J. W. Niemantsverdriet,*-+ I. C. M. Wehman-Ooyevaar,t D. M. Grove,* and G. van Koten***
Sood, A.; Spielvogel, B. F. Main Group Mer. Chem. 1989, 12, 143-148. Gibson, D. H.; Ahmed,F. V.; Phillips, K. R. J . Orgammer. Chem. 1981, 218, 325-336. Soai, K.;Oyamada, H.; Takase, M. Bull. Chem. Soc. Jpn. 1984, 57,
2327-2328. Hall, I. H.,Division of Medicinal Chemistry and Natural Products, School of Pharmacy, University of North Carolina, private communi-
cations. Serum choleserol was determined by the Liebermann-Burchard reaction (Ness, A. T.; Pastewka, J. V.; Peacock, A. C. Clin. Chim. Acta 1964, 10, 229-237).
The antiinflammatoryactivity was measured according to the modified method of Winter (Winter, C. A.; Risley, E.A.; Nus, G. W. froc. Soc. Exp. Biol. Med. 1962, I l l , 544. Hendershot, L. C.; Forsaith, J. J . Pharmacol. Exp. Ther. 1910, 175, 435).
0020-1669/92/ 1331-2655$03.00/0
Received August 8. I991 Introduction Continuing our investigations of oxidative-addition reactions, we have studied in detail the reactions of dihalogens, alkyl halides, and organotin(1V) complexes with various Pt(I1) complexes containing the terdentate monoanionic ligand [CsH3-
'Eindhoven University of Technology. 'University of Utrecht.
0 1992 American Chemical Society
